The use of optical fiber for long-distance transmission of voice and/or data is now common. As the demand for data carrying capacity continues to increase, there is a continuing need to utilize the bandwidth of existing fiber-optic cable more efficiently. An established method for increasing the carrying capacity of existing fiber cable is Wavelength Division Multiplexing (WDM) in which multiple information channels are independently transmitted over the same fiber using multiple wavelengths of light. In this practice, each light-wave-propagated information channel corresponds to light within a specific wavelength range.
In this specification, these individual information-carrying lights of a WDM optical fiber, optical line or optical system are referred to as either “signals” or “channels.” The totality of multiple combined signals, wherein each signal is of a different wavelength range, is herein referred to as a “composite optical signal.” Although each information-carrying channel actually comprises light of a certain range of physical wavelengths, for simplicity, a single channel is referred to as a single wavelength, λ, and a plurality of n such channels are referred to as “n wavelengths” denoted λ1-λn. The symbols λi, λ′i distinguish between channels having the same particular physical wavelength or wavelength range, but possibly carrying different information content.
Conventional OADM systems can separate a composite optical signal into its component channels, remove or “drop” certain selected channels, replace or “add” new channels comprising the same wavelengths as the dropped channels, and re-combine the added channels together with the non-dropped channels into a new composite optical signal. Unfortunately, in conventional OADM systems, which utilize optical filters, a high insertion loss can occur in the separation or “de-multiplexing” stage and the combination or “multiplexing” stage. Each filter has a certain associated insertion loss. The overall insertion loss (in logarithmic units) is the summation of the individual insertion losses associated with each component. Therefore, insertion losses increase for channels encountering more optical components. The losses are greatest for the last channel separated and the first channel added. The losses can become prohibitively large if the optical system employing such an OADM is upgraded so as to include additional channels.
Accordingly, what is needed is an improved upgradeable OADM system. The system should be easily modified or upgraded to accommodate additional channels and should not cause prohibitively large signal losses as a result of such upgrading. The present invention addresses such a need.